This application is based on Japanese Patent Application Nos. 2000-159753 filed on May 30, 2000, and 2001-137262 filed on May 8, 2001.
1. Field of the Invention
The present invention relates to a fluid flow meter having a thermal flow sensor, suitable for use in an intake pipe of vehicle internal combustion engine.
2. Description of Related Art
An air flow meter measuring intake air flow amount of a vehicle engine uses a thermal flow sensor including a heating resistor. The air flow meter detects a change of heat absorbed by the air-flow from a portion where the heating resistor heats, or a change of temperature in the vicinity of the heated portion, thereby attaining the air flow amount.
Since the heat capacity of the thermal flow sensor is small, air-flow turbulence might cause a measurement error. U.S. Pat. No. 5,631,417 discloses an air flow meter in which a fluid passage provided with a thermal flow sensor is smoothly restricted to reduce a turbulence of the air flowing toward the thermal flow sensor. JP-A-10-293052 discloses an air flow meter in which a detecting pipe is disposed within a fluid passage and a supporter into which a thermal flow sensor is installed is disposed within the detecting pipe. The width of the supporter gradually increases from an end in an air-flow direction to a position where the thermal flow sensor is installed, for reducing a turbulence of the air flowing toward the thermal flow sensor.
In general, relations between the air flow amount and the heat absorbed by the air flow from the portion heated by the heating resistor, and the air flow amount and the change of temperature around the heated portion are not linear. Further, response of change of the heat absorbed by the air-flow from the heated portion, and response of the temperature around the heated portion are delayed with respect to the change of air-flow amount. Thus, when the air-flow pulsates due to high-load operation of the engine, measured flow amount average might be smaller than actual air-flow amount average. Here, in case of the thermal flow sensor is installed in a vehicle intake air apparatus such as air-cleaner, error amount of the measured flow amount average varies in accordance with a shape of fluid passage and a disposed position of the air flow meter, and the measured flow amount average deviates from the actual measured flow amount average to larger or smaller than the actual measured flow amount.
In the air flow meter disclosed in the U.S. Pat. No. 5,631,417, since the fluid passage to which the thermal flow sensor is disposed is restricted, flow passage loss thereof is increased. Thus, when the air-flow pulsates, measured flow amount average tends to be smaller than actual flow amount average. In the air flow meter disclosed in JP-A-10-293052, since the supporter into which the thermal flow sensor is installed is disposed within the detecting pipe, flow passage loss thereof is increased. Thus, when the air-flow pulsates, measured flow amount average tends to be smaller than actual flow amount average.
An object of the present invention is to measure fluid flow amount highly accurately even when the fluid flow pulsates.
According to the present invention, a flow passage resistance difference generating member is provided at an inner wall of at least one of first and second sub fluid passages. The flow passage resistance difference generating member works as a flow resistor for the fluid flowing through at least one of the first and second sub fluid passages. Flow passage losses of the first and second sub fluid passages are different from each other due to the flow passage resistance difference generating member. Thus, measured flow amount average when the fluid flow pulsates is corrected to be either larger or smaller by adjusting the flow passage losses of the first and second sub fluid passages by changing size and/or shape of the flow passage resistance difference generating member without changing length and/or fluid passage area of the sub fluid passages. In this way, the actual flow amount is highly accurately measured even when the fluid flow pulsates.